1. Field of the Invention
The present invention relates to a waveguide hybrid junction and, more particularly, to a waveguide hybrid junction serving as a short-slot type directional hybrid junction.
2. Description of the Prior Art
As shown in prespective view of FIG. 1, a conventional waveguide hybrid junction is constituted by a waveguide 10 which is prepared by arranging two rectangular waveguides parallel through one side wall surface and has a small coupling section 21 formed by partially cutting the side wall surface. The waveguide 10 has four terminals 1, 2, 3, and 4 as directional coupling I/O terminals. A waveguide hybrid junction with this arrangement is generally called a short-slot directional hybrid junction.
The basic operation of this waveguide hybrid junction will be explained by dividing its area into three areas 1 to 3, i.e., the area of the coupling section 21 and areas before and after the coupling section 21, as shown in FIG. 2.
First, when a radio wave of TE.sub.10 mode is excited at the terminal 1 of the area 1, radio waves of TE.sub.10 and TE.sub.20 modes are excited in the area 2. If a length L of the coupling section 21 (area 2) is so selected as to obtain a phase shift difference of about 90.degree. between the TE.sub.10 and TE.sub.20 modes, radio waves of TE.sub.10 mode having almost the same amplitude value and a phase shift difference of about 90.degree. are excited at the terminals 3 and 4. As a result, in the waveguide hybrid junction, for example, a radio wave incident from the terminal 1 is output to not the terminal 2 but the terminals 3 and 4, and a radio wave incident from the terminal 3 is similarly output to the terminals 1 and 2.
The frequency vs. phase shift characteristics and amplitude characteistics of this waveguide hybrid junction will be described below.
Of parameters S between these four terminals, S.sub.31 represents coupling from the terminal 1 to the terminal 3, and S.sub.41 represents coupling fromthe terminal 1 to the terminal 4. Under perfect match conditions, S.sub.31 and S.sub.41 are given by the following equations: ##EQU1##
A phase shift difference between radio waves at the terminals 3 and 4 input from the terminal 1 is expressed by .THETA.. ##EQU2## where .beta..sub.3 (z) and .beta..sub.4 (z) are phase constants in the TE.sub.10 and TE.sub.20 modes at a coupling portion A, respectively.
In the above equations, .theta..sub.3 and .theta..sub.4 represent propagation phase shift amounts in TE.sub.10 and TE.sub.20 modes at the coupling portion 21, respectively.
First, the phase shift characteristics will be described.
FIG. 3A is a graph showing the frequency characteristic of a difference .DELTA.=.theta..sub.3 -.theta..sub.4 (solid line) between the phase shift amounts at the coupling section 21 (to be referred to as the coupling portion A hereinafter) of the waveguide hybrid junction having the shape shown in FIG. 1, and that of a difference 2.DELTA..phi.=2 (.phi..sub.13 -.phi..sub.14) (broken line) between the phase shift amounts at discontinuous portions 22 and 23 (to be referred to as discontinuous portions B and B' hereinafter). As described above, the length L of the coupling portion A is selected such that .DELTA..theta.=.theta..sub.3 -.DELTA..sub.4 becomes almost 90.degree. within the frequency range of f.sub.1 to f.sub.2 as a target range of this waveguide hybrid junction, as shown in FIG. 3A.
.phi..sub.13 and .phi..sub.14 represent phase shift amounts in the TE.sub.10 and TE.sub.20 modes, respectively. A difference between the phase shift amounts in the TE.sub.10 and TE.sub.20 modes generated at the corresponding discontinuous portions B and B' is given by .DELTA..phi.=.phi..sub.13 -.phi..sub.-.
A radio wave input from the terminal 1 is output to the terminal 4 through the two discontinuous portions (B and B'). For this reason, the difference between the phase shift amounts in the TE.sub.10 and TE.sub.20 modes generated at the discontinuous portions between the input and output of the short-slot hybrid is 2.DELTA..phi.. The characteristic indicated by the broken line in FIG. 3A is obtained.
The phase shift difference .THETA. generated when radio waves of the respective modes input from the terminal 1 are output to the terminals 3 and 4 is calculated from a difference between the phase shift difference .DELTA..theta.generated at the coupling portion A and the phase shift difference 2.increment..phi.generated at the discontinuous portions B and B',i.e., .THETA.=.DELTA..theta.-2.increment..THETA..
FIG. 3B is a graph showing the frequency characteristics of .THETA. obtained by this calculation. As is apparent from FIG. 3B, the phase shift difference is almost 90.degree. within the frequency band of f.sub.1 to f.sub.2.
Next, the amplitude characteristics will be described.
An amplitude characteristic .vertline.S.sub.31 .vertline. for coupling from the terminal 1 to the terminal 3 and an amplitude characteristic .vertline.S.sub.41 .vertline. for coupling from the terminal 1 to the terminal 4 are obtained by substituting .THETA. prepared by the above calculation into equations (1) and (2), respectively. The frequency characteristics of these amplitude characteristics are shown in FIG. 4.
Referring to FIG. 4, both the amplitude characteristics .vertline.S.sub.31 .vertline. and .vertline.S.sub.41 .vertline. have a loss of about -3 dB within the limited frequency band of f.sub.1 to f.sub.2, and a signal input from the terminal 1 is distributed almost half and half to the terminals 3 and 4.
The conventional waveguide hybrid junction described above is shown in, e.g., reference: Fumikazu Oguchi "Microwave and Millimeter Wave", pp. 303-305.
The conventional waveguide hybrid junction has a compact, relatively simple structure. Further, good characteristics can be ensured over a relatively broad band.
Referring to FIGS. 3B and 4, the amplitude and phase shift characteristics respectively have a loss of about 3 d B and a phase shift difference of almost 90.degree. within the limited frequency band of f.sub.1 to f.sub.2, as described above. However, at, e.g., a frequency f.sub.1 ' lower than the frequency f.sub.1 in FIG. 4, the distribution ratio of the amplitude characteristics .vertline.S.sub.31 .vertline. and .vertline.S.sub.41 .vertline. greatly differs from -3 d B. Also in FIG. 3B, the phase shift difference .THETA. greatly differs from 90.degree. in the frequency band of f.sub.1 ' to f.sub.1.
In this manner, although the conventional waveguide hybrid junction exhibits good characteristics within a frequency band determined by the shape of the waveguide, it greatly degrades at a lower frequency and therefore cannot be used. In particular, transmission of multimedia signals, transmission of broad-band ISDN signals, and the like are requiring waveguide hybrid junctions with better characteristics. The above degradation in signal characteristics in a low frequency band poses a problem.